(Issue 3402) Add work-around for iOS 7 Safari bug impacting bootstrap modal

Turns out this is a bug in iOS 7 Safari: https://github.com/twbs/bootstrap/issues/9023

Basically Safari is not updating the position of fixed elements after it expands / collapses the keyboard (or other items such as the tool bar).

I've resolved this issue by using a posted work around, which tests to work well, though is a bit of a hack. I'm not aware of any better solutions to the problem.